docs: def main(host):
      client = capnp.TwoPartyClient(host)
  
      # Pass "calculator" to ez_restore (there's also a `restore` function that
      # takes a struct or AnyPointer as an argument), and then cast the returned
      # capability to it's proper type. This casting is due to capabilities not
      # having a reference to their schema
      calculator = client.bootstrap().cast_as(calculator_capnp.Calculator)
  
      '''Make a request that just evaluates the literal value 123.
  
      What's interesting here is that evaluate() returns a "Value", which is
      another interface and therefore points back to an object living on the
      server.  We then have to call read() on that object to read it.
      However, even though we are making two RPC's, this block executes in
      *one* network round trip because of promise pipelining:  we do not wait
      for the first call to complete before we send the second call to the
      server.'''
  
      print('Evaluating a literal... ', end="")
  
      # Make the request. Note we are using the shorter function form (instead
      # of evaluate_request), and we are passing a dictionary that represents a
      # struct and its member to evaluate
      eval_promise = calculator.evaluate({"literal": 123})
  
      # This is equivalent to:
      '''
      request = calculator.evaluate_request()
      request.expression.literal = 123
  
      # Send it, which returns a promise for the result (without blocking).
      eval_promise = request.send()
      '''
  
      # Using the promise, create a pipelined request to call read() on the
      # returned object. Note that here we are using the shortened method call
      # syntax read(), which is mostly just sugar for read_request().send()
      read_promise = eval_promise.value.read()
  
      # Now that we've sent all the requests, wait for the response.  Until this
      # point, we haven't waited at all!
      response = read_promise.wait()
      assert response.value == 123
  
      print("PASS")
  
      '''Make a request to evaluate 123 + 45 - 67.
  
      The Calculator interface requires that we first call getOperator() to
      get the addition and subtraction functions, then call evaluate() to use
      them.  But, once again, we can get both functions, call evaluate(), and
      then read() the result -- four RPCs -- in the time of *one* network
      round trip, because of promise pipelining.'''
  
      print("Using add and subtract... ", end='')
  
      # Get the "add" function from the server.
      add = calculator.getOperator(op='add').func
      # Get the "subtract" function from the server.
      subtract = calculator.getOperator(op='subtract').func
  
      # Build the request to evaluate 123 + 45 - 67. Note the form is 'evaluate'
      # + '_request', where 'evaluate' is the name of the method we want to call
      request = calculator.evaluate_request()
      subtract_call = request.expression.init('call')
      subtract_call.function = subtract
      subtract_params = subtract_call.init('params', 2)
      subtract_params[1].literal = 67.0
  
      add_call = subtract_params[0].init('call')
      add_call.function = add
      add_params = add_call.init('params', 2)
      add_params[0].literal = 123
      add_params[1].literal = 45
  
      # Send the evaluate() request, read() the result, and wait for read() to finish.
      eval_promise = request.send()
      read_promise = eval_promise.value.read()
  
      response = read_promise.wait()
      assert response.value == 101
  
      print("PASS")
  
      '''
      Note: a one liner version of building the previous request (I highly
      recommend not doing it this way for such a complicated structure, but I
      just wanted to demonstrate it is possible to set all of the fields with a
      dictionary):
  
      eval_promise = calculator.evaluate(
  {'call': {'function': subtract,
            'params': [{'call': {'function': add,
                                 'params': [{'literal': 123},
                                            {'literal': 45}]}},
                       {'literal': 67.0}]}})
      '''
  
      '''Make a request to evaluate 4 * 6, then use the result in two more
      requests that add 3 and 5.
  
      Since evaluate() returns its result wrapped in a `Value`, we can pass
      that `Value` back to the server in subsequent requests before the first
      `evaluate()` has actually returned.  Thus, this example again does only
      one network round trip.'''
  
      print("Pipelining eval() calls... ", end="")
  
      # Get the "add" function from the server.
      add = calculator.getOperator(op='add').func
      # Get the "multiply" function from the server.
      multiply = calculator.getOperator(op='multiply').func
  
      # Build the request to evaluate 4 * 6
      request = calculator.evaluate_request()
  
      multiply_call = request.expression.init("call")
      multiply_call.function = multiply
      multiply_params = multiply_call.init("params", 2)
      multiply_params[0].literal = 4
      multiply_params[1].literal = 6
  
      multiply_result = request.send().value
  
      # Use the result in two calls that add 3 and add 5.
  
      add_3_request = calculator.evaluate_request()
      add_3_call = add_3_request.expression.init("call")
      add_3_call.function = add
      add_3_params = add_3_call.init("params", 2)
      add_3_params[0].previousResult = multiply_result
      add_3_params[1].literal = 3
      add_3_promise = add_3_request.send().value.read()
  
      add_5_request = calculator.evaluate_request()
      add_5_call = add_5_request.expression.init("call")
      add_5_call.function = add
      add_5_params = add_5_call.init("params", 2)
      add_5_params[0].previousResult = multiply_result
      add_5_params[1].literal = 5
      add_5_promise = add_5_request.send().value.read()
  
      # Now wait for the results.
      assert add_3_promise.wait().value == 27
      assert add_5_promise.wait().value == 29
  
      print("PASS")
  
      '''Our calculator interface supports defining functions.  Here we use it
      to define two functions and then make calls to them as follows:
  
        f(x, y) = x * 100 + y
        g(x) = f(x, x + 1) * 2;
        f(12, 34)
        g(21)
  
      Once again, the whole thing takes only one network round trip.'''
  
      print("Defining functions... ", end="")
  
      # Get the "add" function from the server.
      add = calculator.getOperator(op='add').func
      # Get the "multiply" function from the server.
      multiply = calculator.getOperator(op='multiply').func
  
      # Define f.
      request = calculator.defFunction_request()
      request.paramCount = 2
  
      # Build the function body.
      add_call = request.body.init("call")
      add_call.function = add
      add_params = add_call.init("params", 2)
      add_params[1].parameter = 1  # y
  
      multiply_call = add_params[0].init("call")
      multiply_call.function = multiply
      multiply_params = multiply_call.init("params", 2)
      multiply_params[0].parameter = 0  # x
      multiply_params[1].literal = 100
  
      f = request.send().func
  
      # Define g.
      request = calculator.defFunction_request()
      request.paramCount = 1
  
      # Build the function body.
      multiply_call = request.body.init("call")
      multiply_call.function = multiply
      multiply_params = multiply_call.init("params", 2)
      multiply_params[1].literal = 2
  
      f_call = multiply_params[0].init("call")
      f_call.function = f
      f_params = f_call.init("params", 2)
      f_params[0].parameter = 0
  
      add_call = f_params[1].init("call")
      add_call.function = add
      add_params = add_call.init("params", 2)
      add_params[0].parameter = 0
      add_params[1].literal = 1
  
      g = request.send().func
  
      # OK, we've defined all our functions.  Now create our eval requests.
  
      # f(12, 34)
      f_eval_request = calculator.evaluate_request()
      f_call = f_eval_request.expression.init("call")
      f_call.function = f
      f_params = f_call.init("params", 2)
      f_params[0].literal = 12
      f_params[1].literal = 34
      f_eval_promise = f_eval_request.send().value.read()
  
      # g(21)
      g_eval_request = calculator.evaluate_request()
      g_call = g_eval_request.expression.init("call")
      g_call.function = g
      g_call.init('params', 1)[0].literal = 21
      g_eval_promise = g_eval_request.send().value.read()
  
      # Wait for the results.
      assert f_eval_promise.wait().value == 1234
      assert g_eval_promise.wait().value == 4244
  
      print("PASS")
  
      '''Make a request that will call back to a function defined locally.
  
      Specifically, we will compute 2^(4 + 5).  However, exponent is not
      defined by the Calculator server.  So, we'll implement the Function
      interface locally and pass it to the server for it to use when
      evaluating the expression.
  
      This example requires two network round trips to complete, because the
      server calls back to the client once before finishing.  In this
      particular case, this could potentially be optimized by using a tail
      call on the server side -- see CallContext::tailCall().  However, to
      keep the example simpler, we haven't implemented this optimization in
      the sample server.'''
  
      print("Using a callback... ", end="")
  
      # Get the "add" function from the server.
      add = calculator.getOperator(op='add').func
  
      # Build the eval request for 2^(4+5).
      request = calculator.evaluate_request()
  
      pow_call = request.expression.init("call")
      pow_call.function = PowerFunction()
      pow_params = pow_call.init("params", 2)
      pow_params[0].literal = 2
  
      add_call = pow_params[1].init("call")
      add_call.function = add
      add_params = add_call.init("params", 2)
      add_params[0].literal = 4
      add_params[1].literal = 5
  
      # Send the request and wait.
      response = request.send().value.read().wait()
      assert response.value == 512
  
      print("PASS")